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300
METALLURGY OF IRON" AND STEEL.
The different photographs in Fig. XV-B represent the appearance of steels of different carbon content. No. 3 is a steel containing 1.39 per cent, of carbon and is from a bar in the condition in which it left the rolls. It shows a pearlite grain surrounded by walls of cementite. Nos. 4 and 5 represent lamellar and granular
TABLE XV-L. Theoretical Micro-Structure of Carbon Steels.
Carbon per cent.	Pearlite.	Fe.	Cera.
0	0	100	0
.10	12	88	0
.40	50	50     .	0
.70	87	13	0
.80	100	0	0
1.00	97	0	3
1.20	93	0	7
2.50	71	0	29
TABLE XY-M. Micro-Structural Composition of some Quenched Carbon Steels.
Carbon, per cent*	Quenched above Ar,		Quenched between Ar3 and Ara.			Quenched between Ara and Ar.^			Quenched below Arj or slowly cooled		
	Mart.   Fer.	Gem..	Mart	Fer.	Cem,	Mart.	Fer.	Cem.	Pearl.	Fer.	Cem.
0.09 0.21 0.35 0.80 1 20 2 50	77       23	0	27	73	0	11 31 56	89 69 44	0 0 0	10 23 50 100 92 77	90 77 .60 0 0 0	0 0 0 0 8 23
	Quenched above Ara.										
	MartensJ be.	Ferrite.		Cementite.							
	100 100	0 0		0 0							
	Quenched above AT]..										
	Martensite;	Ferrite.		Cementite.							
	500 94 SO	0 0 0    .		0 6 20							
pearlite respectively. No. 6 is a steel containing .67 per cent, of carbon, the appearance of -which is similar-to No. 3, but there is really quite a difference, in that there is not a sufficient amount of carbon to' form the eutectic alloy. -Consequently there is an excess of ferrite and this forms the walls, whereas when the carbon ex-